1. Technical Field of the Invention
The present invention relates to ignition coils, and more particularly, to an ignition coil which incorporates an igniter to apply a high voltage to an ignition plug.
2. Description of Related Art
For example, as an ignition coil of the type incorporating an igniter, an ignition coil which employs electrically insulating oil to ensure insulation is introduced in Japanese Patent Laid-Open Publication No. Hei 7-153636. A circuit module containing a power transistor, a primary coil portion, a secondary coil portion, and the like are accommodated inside the housing of the ignition coil in the publication. The housing also has a liquid electrical insulating oil injected therein. The electrical insulating oil ensures insulation between the aforementioned members.
On the other hand, an ignition coil which employs an epoxy resin to ensure insulation is introduced in Japanese Patent Laid-Open Publication No. Hei 2001-127239. An igniter, a primary coil portion, a secondary coil portion, and the like are accommodated inside the housing of the ignition coil described in the publication. The igniter is also formed of a heat sink, a hybrid integrated circuit, a power transistor, and the like, which are encapsulated in a molding resin.
FIG. 8 is an enlarged cross-sectional view showing the vicinity of the igniter in the ignition coil described in the publication. An igniter 100 comprises a heat sink 101, a power transistor 102, a hybrid integrated circuit 103, and an igniter terminal 104. The igniter 100 is placed on a mount 113. The power transistor 102 and the hybrid integrated circuit 103 are secured to the heat sink 101. An aluminum wire 106 electrically connects between the power transistor 102 and the hybrid integrated circuit 103. An aluminum wire 107 electrically connects the hybrid integrated circuit 103 and the igniter terminal 104. The hybrid integrated circuit 103 is covered with silicone rubber 105. These members are encapsulated in a molding resin 108 with the top end of the igniter terminal 104 protruded. The molding resin 108 forms an outer shell of the igniter 100. The igniter terminal 104 is jointed to a connector terminal 110, which lies in a connector 109. The connector terminal 110 is electrically connected to an engine control unit ECU. A housing 112 forming the outer shell of the ignition coil has an epoxy resin 111 injected therein to ensure insulation between the members accommodated inside the housing 112 and to secure each member. The housing 112 has the epoxy resin 111 filled and hardened in between the members therein.
However, the ignition coil described in Japanese Patent Laid-Open Publication No. Hei 7-153636, which ensures insulation by using the electrically insulating oil, raises the following problems. That is, the electrically insulating oil is a liquid. For this reason, a robust sealing mechanism is required to prevent the electrically insulating oil from leaking out of the ignition coil. This causes an increase in complexity of the structure of the ignition coil due to the sealing mechanism. This also causes an increase in the size of the ignition coil. This further causes an increase in manufacturing costs.
On the other hand, the ignition coil described in Japanese Patent Laid-Open Publication No. Hei 2001-127239, which ensures insulation by using the epoxy resin, raises the following problems. That is, double layers of a resin cover the heat sink 101, the power transistor 102, the hybrid integrated circuit 103, the silicone rubber 105, the aluminum wire 106, the aluminum wire 107, and part of the igniter terminal 104. That is, these members are doubly covered with the molding resin 108 and the epoxy resin 111. Accordingly, upon fabrication of the ignition coil, this requires the additional and independent steps of encapsulating the aforementioned members in the molding resin 108 to fabricate the igniter 100, and placing the fabricated igniter 100 on the mount 113 to inject and harden the epoxy resin 111. Accordingly, this increases the complexity of the fabrication process. This also causes an increase in manufacturing costs. This further causes an increase in complexity of the structure of the ignition coil due to the provision of the double resin layers. This further causes double electrical connections with the outside.
The ignition coil according to the present invention was completed in view of the aforementioned problems. It is therefore an object of the present invention to provide an ignition coil that can be fabricated at low costs in a simple structure and reduced in size.